Conventional vapor compression systems typically include a compressor, a heat rejection heat exchanger, a heat absorption heat exchanger, and expansion device, commonly an expansion valve, disposed upstream with respect to working fluid flow, of the heat absorption heat exchanger and downstream of the heat rejection heat exchanger. These basic system components are interconnected by working fluid lines in a closed circuit, arranged in accord with known vapor compression cycles.
In some vapor compression systems, capacity modulation capability may be added by incorporating a flash tank economizer is into the working fluid circuit between the heat rejection heat exchanger and the evaporator. In such case, the working fluid leaving the heat rejection heat exchanger is expanded through an economizer expansion device, such as a thermostatic expansion valve or an electronic expansion valve, prior to entering the flash tank wherein the expanded fluid separates into a liquid component and a vapor component. The vapor component is thence directed from the flash tank into an intermediate pressure stage of the compression process of a multi-stage compression device, while the liquid component is directed from the flash tank through the system's main expansion valve prior to entering the evaporator.
Depending upon the characteristics of the working fluid in use in a particular application, a vapor compression system may be operated in either a subcritical mode or a transcritical mode. In vapor compression systems operating in a subcritical cycle, both the vapor heat rejection heat exchanger and the heat absorption heat exchanger operate at pressures below the critical pressure of the working fluid. Thus, in the subcritical mode, the vapor heat rejection heat exchanger functions as a working fluid condenser and the heat absorption heat exchanger functions as a working fluid evaporator.
However, in refrigerant vapor compression systems operating in a transcritical cycle, the vapor heat rejection heat exchanger operates at a refrigerant temperature and pressure in excess of the refrigerant's critical pressure, while the heat absorption heat exchanger operates at a refrigerant temperature and pressure in the subcritical range. Thus, in the transcritical mode, the vapor heat rejection heat exchanger functions as a working fluid gas cooler and the heat absorption heat exchanger functions an as a working fluid evaporator.
In vapor compression systems used in refrigeration applications, commonly referred to as refrigerant vapor compression systems, the working fluid is refrigerant. Refrigerant vapor compression systems charged with conventional refrigerants, such as for example, fluorocarbon refrigerants such as, but not limited to, hydrochlorofluorocarbons (HCFCs), such as R22, and more commonly hydrofluorocarbons (HFCs), such as R134a, R404A, and R407C, typically operate in the subcritical mode. “Natural” refrigerants, such as carbon dioxide, are also used in refrigerant vapor compression systems instead of HCFC or HFC refrigerants. Because carbon dioxide has a low critical temperature, most refrigerant vapor compression systems charged with carbon dioxide as the refrigerant are designed for operation in the transcritical mode.
Refrigerant vapor compression systems are commonly used for conditioning air to be supplied to a climate controlled comfort zone within a residence, office building, hospital, school, restaurant or other facility. Refrigerant vapor compression system are also commonly used for refrigerating air supplied to display cases, merchandisers, freezer cabinets, cold rooms or other perishable/frozen product storage areas in commercial establishments. Refrigerant vapor compression systems are also commonly used in transport refrigeration systems for refrigerating air supplied to a temperature controlled cargo space of a truck, trailer, container or the like for transporting perishable/frozen items by truck, rail, ship or intermodal.
Refrigerant vapor compression systems used in connection with transport refrigeration systems are generally subject to more stringent operating conditions than in air conditioning or commercial refrigeration applications due to the wide range of operating load conditions and the wide range of outdoor ambient conditions over which the refrigerant vapor compression system must operate to maintain product within the cargo space at a desired temperature. The desired temperature at which the cargo needs to be controlled can also vary over a wide range depending on the nature of cargo to be preserved. The refrigerant vapor compression system must not only have sufficient capacity to rapidly pull down the temperature of product loaded into the cargo space at ambient temperature, but also operate efficiently at low load when maintaining a stable product temperature during transport. Additionally, transport refrigerant vapor compression systems are subject to cycling between an operating mode and standstill mode, i.e. an idle state.
In more complex refrigeration vapor compression systems, such as those equipped with a multi-stage compression device and capacity modulation, it is customary to provide a number of refrigerant flow control devices to permit selective control of refrigerant flow through the various branches of the refrigerant circuit. In operation of conventional refrigerant vapor compression systems, it is customary practice to position each flow control device in the refrigerant vapor compression system in a fully closed position during standstill that is when the refrigerant vapor compression system is idle.
With the flow control devices fully closed, the potential exists for refrigerant to be trapped in isolated pockets of the refrigerant circuit between fully closed flow control devices. If refrigerant becomes trapped in an isolated pocket during standstill, the pressure within the isolated pocket may increase to a level in excess of the design containment pressure of the tube, tank or other structure in which the refrigerant is resident, particularly on the low-pressure side of the refrigerant vapor compression system. If the refrigerant pressure within an isolated pocket does exceed the design containment pressure, cracks could potentially develop in the containment structure resulting in refrigerant leaking from the system.